Add support for $anchor

[lordpixel23]

JSON Schema now includes a mechanism to simplify references as described in Section 9.2 of 2020-12 draft.

https://json-schema.org/draft/2020-12/json-schema-core.html#rfc.section.9.2

Basically defining named anchors and referring to them rather than using JSON pointer.

Currently this does not work, returning (in Swagger CLI at least) printing
Invalid $ref pointer "fn". Pointers must begin with "#/"

[philsturgeon]

Thanks for the heads up. This is all covered in #145 and will be the basis of an already started rewrite!